The present invention relates, in general, to the measurement of hemodynamic parameters by indicator dilution, and more particularly to measurements of blood flow, blood volume, cardiac output, protein concentration, lung water, and solution clearance through the measurement of the sound velocity dilution effects of an indicator in the bloodstream of a patient.
In a large number of medical procedures, it is very important to monitor with precision hemodynamic parameters in a patient. For example, in procedures such as hemodialysis, a filter is used to remove selected particles and liquids from the patient's bloodstream, but if this material is removed too quickly, the vascular system of the patient may collapse.
Medical parameters such as protein concentration in the blood, cardiac output, lung water content, blood circulating volume, and the like also require careful measurement and monitoring, and improved accuracy in such measurements is highly desirable.
Difficulties in making such measurements have resulted from the fact that such procedures usually involve extracorporeal circulation of the blood from a patient through, for example a blood treatment system, and in many cases the effects of the system itself on the blood flow or on the measurement devices is unknown. For example, if blood is directed to a dialysis filter through plastic tubing, the effect of the plastic material on measuring equipment using ultrasound waves may not be known with any certainty, since the characteristics of the material can vary from one tube to another. Difficulties are also encountered in perivascular measurements, i.e., measurements made using sensors mounted on a blood vessel, for the thickness of a vessel wall can adversely affect the measurement. Even when measurements are being made in the patient's body, as by an ultrasonic blood flow meter clamped around a blood vessel, the thickness and morphology of the vein/artery will affect the measurement.